The present invention relates to a fuel flow control method and a fuel flow control system of a gas turbine combustor provided in a humid air gas turbine.
JP-A-2008-175098 discloses fuel control means capable of maintaining flame stability before and after the starting of humidification while ensuring a low NOx performance of a combustor in a humid air gas turbine power plant, which achieves an improvement in output and efficiency through adding moisture to a gas turbine working fluid (air) to humidify the same and using the humidified air to recover heat energy stored in gas turbine exhaust gases.
Generally, when the number of revolution rises at the startup of a gas turbine, an operating state tends to become unstable due to disturbance as compared with that after a rated number of revolution is reached, since a compressor intake air flow rate and the vibration characteristic of a rotating body vary.
In humid air gas turbine power plants, since disturbance is caused on a gas turbine when humidification is started in the course of an increase in number of revolution, it is desirable to start humidification in a partially loaded state after a rated number of revolution is reached, in order to ensure stability at the startup.
A major part of NOx generated in a combustor comprises thermal NOx generated by oxidation of nitrogen contained in air when fuel, such as natural gas, kerosene, light oil, or the like, having a small nitrogen content is used.
Since generation of thermal NOx is high in temperature dependency, the basic idea of a low NOx combustion method in gas turbines, in which such fuel is used, resides generally in a decrease in flame temperature. Premixed combustion, in which fuel and air are beforehand mixed and then burned, is known as a measure for a decrease in flame temperature.
Also, in the case where combustion air is made high in temperature by a regenerator as in a humid air gas turbine plant, it is necessary to attain low NOx through appropriately controlling flame temperature while preventing self-ignition of fuel, and a method, shown in JP-A-2008-175098, of jetting fuel and air as a multiplicity of coaxial jet streams of small diameter into a combustion chamber is effective.
In order to make a low NOx performance and flame compatible with each other in such low NOx combustor, it is essential to regulate a fuel-air ratio, which is a ratio of fuel flow rate and air flow rate, in a predetermined range.
JP-A-7-189743 discloses means of regulating a ratio of fuel flow rate and air flow rate aiming at a change in opening degree of a compressor inlet guide valve, which accompanies operation of a gas turbine, a change in atmospheric temperature, a change in air flow rate, which is attributable to a change in atmospheric pressure, and a change in fuel flow rate, which is attributable to fuel temperature and a change in fuel heating value.
JP-A-11-72029 discloses means of achieving an increase and a decrease in fuel flow rate in accordance with humidity of atmospheric intake air and intake spraying water quantity, in a gas turbine system, in which intake air of a compressor is cooled by intake spraying for reduction in compressive power.
When humidification is started in a humid air gas turbine power plant, combustion air in a combustor is increased in humidity, so that heat of combustion is deprived of to lead to a decrease in flame temperature and a NOx yield is reduced.
Also, since addition of moisture causes a turbine working fluid to increase in flow rate, fuel flow rate is decreased in order to maintain the number of revolution constant. Reduction in fuel flow rate leads to a decrease in flame temperature, so that a NOx yield decreases. Further, since a decrease in flame temperature leads to reduction in recovered heating value in a regenerator, combustion air temperature drops. A decrease in combustion air temperature causes flame temperature to be decreased to lead to reduction in NOx yield.
Humidification is started in this manner whereby (1) an increase in moisture, (2) reduction in fuel flow rate, and (3) a decrease in air temperature advance at the same time to lead to a decrease in flame temperature, so that a NOx yield decreases but combustion stability is degraded.
Setting combustion air flow rate low taking previous account of humidification enables eliminating occurrence of flame blow-off under a high humid condition. However, in a combustor with combustor air distribution thus set, flame temperature rises conversely to the matter described above before the starting of humidification, so that flame stability is ensured but a NOx yield tends to increase.
That is, in a humid air gas turbine plant, before and after the starting of humidification, NOx generation and flame stability in a combustor suffer a large change in condition. Also, it is thought that at the time of an increase in gas turbine load, lag is caused by valve control and a volume of an associated system until moisture is actually added to combustion air after the starting of humidification.
It is thought that at the time of a decrease in gas turbine load, lag is caused due to the same reason until combustion air is decreased in humidity.
When in starting the humidifying operation and stopping the operation, combustion air humidity is varied after a lag time, flame temperature possibly rises or drops excessively, so that there is a possibility of a remarkable increase in NOx and a decrease in combustion stability.
Accordingly, there is a demand for control means for stable combustion of a combustor in low NOx under such change in condition.
Hereupon, as disclosed in JP-A-2008-175098, combustion stability after humidification can be ensured with the use of means for setting a ratio of fuel supplied to a combustion section of an excellent flame holding performance to control fuel so that a part of combustion sections of a combustor provided with a plurality of combustion sections supplied individually with fuel comprises combustion section or sections (provided with air holes, which impart swirl components to air flow), which are more excellent in flame holding performance than the remaining combustion sections and for a predetermined period of time after the starting of humidification, combustion temperature in the combustion section or sections having an excellent flame holding performance is made equal to or higher than combustion temperature before the starting of humidification.
In the case where in accommodating such change in humidity, the means as disclosed in JP-A-7-189743 is applied to ensure stability in combustion, it is thought to measure moisture in combustion air to control a ratio of fuel flow rate on the basis of the value thereof.
Hereupon, it is thought to measure moisture in combustion air with the use of a humidity sensor.
In view of a humidity measuring position, humidity measurement at an outlet of a humidifier is first taken into consideration. Since air at an outlet of a humidifier is close to the dew point, however, there is caused a problem that accuracy of measurement cannot be expected in measurement with a humidity sensor. Secondly, humidity measurement at an outlet of a regenerator is taken into consideration. Since air at an outlet of a regenerator is as high as 450° C. or above, however, high heat resistance is required of a humidity sensor.
Subsequently, a performance demanded of a humidity sensor is taken into consideration. Due to a change in humidity contained in air, a combustion state varies every moment. Therefore, it is demanded of a humidity sensor to measure humidity in air with high responsibility to control a ratio of fuel flow rate to maintain stable combustion.
Thus, there are caused many problems in measuring humidity in air with the use of a humidity sensor to exercise combustion control to achieve stable combustion.
JP-A-11-72029 discloses means of achieving stable combustion accommodating that change in combustion air humidity, which is caused by a change in atmospheric humidity and intake spraying water quantity, in a gas turbine system, in which intake air of a compressor is cooled by an intake spraying device for reduction in compressive power.
Since the humid air gas turbines disclosed in the prior art comprise a humidifier positioned downstream of a compressor, a large change in combustion air humidity is caused by not only intake spraying with the compressor but also humidification with the humidifier. Also, when compressor discharge air is varied in temperature and humidity depending upon the operating condition of an intake spraying device, it is thought that the humidifier is varied in humidification in accordance therewith.
However, no examination has been made for the technology of controlling a fuel flow rate of a gas turbine combustor taking account of both a change in humidity in a compressor due to intake spraying and a change in humidity in a humidifier in order to cause stable combustion in the gas turbine combustor of a humid air gas turbine.
Also, no examination has been made for the technology of controlling a fuel flow rate of a humid air gas turbine combustor to be capable of operation in high reliability before humidification, before and after the starting of humidification, and during humidification in a humid air gas turbine without damage in combustion stability of a gas turbine combustor and of maintaining a NOx yield in low level irrespective of a humidified condition.